1. Field of the Invention
This invention relates to a write device for an optical printer, and more particularly to an optical write head which irradiates dot-like light selectively onto a record medium of the printer by means of an electro-optical shutter for interrupting or transmitting dot-like light emitted from a light source.
2. Description of the Prior Art
Recently, various kinds of write device for an optical printer have been proposed. FIG. 5 schematically illustrates typical one of such optical printers. In FIG. 5, reference numeral 1 designates a photosensitive drum which acts as a record medium and is rotated in a clockwise direction as shown by the arrow in FIG. 5 so that its surface may be charged by an electrifier 2. The surface of the photosensitive drum 1 is irradiated by dot-like light from a write device 3 to form a latent image of characters, figures or the like thereon. The latent image is then developed by a developing device 4. Decalcomania paper 6 contained in a cassette 5 is fed between the surface of the photosensitive drum 1 and a heater 7 so as to continuously and thermally transfer characters, figures or the like formed on the surface of the photosensitive drum 1 to the decalcomania paper 1. In FIG. 5, reference numeral 8 designates an erasing lamp for erasing the transferred image and 9 is a cleaning blade for cleaning the surface of the photosensitive drum 1.
Conventionally, an LED (light emitting diode), a CRT (cathode ray tube) or the like has been used as a write head 3 for such an optical printer. Also, an optical write head has been recently developed which uses the principle of a vacuum fluorescent display device in addition to the light-emission devices such as LED and the like in view of the reliability and the manufacturing costs.
A fluorescent display device is a kind of multi-electrode electron tube which includes a casing evacuated to high vacuum in which various electrodes and phosphor layers are contained. For example, the fluorescent display device is constructed in such a manner that anode conductors each having the phosphor layer deposited thereon are arranged on an inner surface of an anode substrate constituting a part of an envelope and electrons emitted from filamentary cathodes are impinged on the phosphor layers of the anode conductors to cause them to emit light for effecting luminous display. The luminous display is then observed through a front cover arranged opposite to the anode substrate. Also, there is known a front-emission type fluorescent display device in which an anode substrate and anode conductors each are made of a transparent material to permit luminous display of phosphor layers to be observed through the anode conductors.
FIG. 4 shows a write head 10 for the optical printer which is of a tetrode-type structure and is disclosed in U.S. Pat. No. 4,701,670. The write head 10 utilizes the principle of such a conventional fluorescent display device as described above.
The write head 10 shown in FIG. 4 comprises a plurality of linear or strip-like anode conductors 12 arranged in parallel to one another on a substrate 11 and a phosphor layer 13 uninterruptedly deposited on each of the anode conductors 12, thereby to form a plurality of anodes 14. Above the anodes 14 are arranged a plurality of second control electrodes 15 through an insulating layer (not shown) provided on a periphery of the substrate 11. The second control electrodes 15 are arranged in parallel to each other and each comprises a flat plate member arranged obliquely across arrangement of the anode conductors 12. Also, the second control electrodes 15 are electrically separated from one another and adapted to cause a printing signal to be supplied thereto. In addition, the second control electrodes 15 each are provided at a central portion thereof with a slit-like opening or slit 16, which is arranged to extend obliquely across arrangement of the anode conductors 12. A portion of each of the phosphor layers 13 defined by each of the slits 16 constitutes a luminous dot 17 which emits light when electrons are impinged thereon. Thus, it is to be notd that the second control electrodes 15 exhibit two functions for forming the luminous dot 17 and for controlling actuation of the luminous dot 17.
At both ends of the second control electrodes 15, insulating spacers (not shown) are provided, which dispose a first control electrode 18 above the second control electrodes 15. The first control electrode 18 is formed of a single sheet-like material which is provided at portions thereof positionally corresponding to the slits 16 of the second control electrodes 15 with slits 19 of a size larger than that of the slits 16. This construction causes only the luminous dots 17 to constitute an effective luminous section, because portions of the phosphor layers 13 of the anodes 14 appearing at gaps between the respective two adjacent second control electrodes 15 are masked by the first control electrode 18. When manufacturing, it is not necessarily required to uninterruptedly deposit the phosphor layers 13 on the anode conductors 12. The phosphor layers 13 may be deposited on at least portions of the anode conductors which are permitted to view through the slits 16 of the second control electrodes 15.
Above the first control electrode 18, filamentary cathodes 20 are stretchedly arranged. Also, a casing assembled by side plates 21 and a front cover 22 is sealedly mounted on the substrate 11 by means of a sealing material so as to constitute an envelope in cooperation with the substrate 11. The envelope is then evacuated to a high vacuum.
From the evacuated envelope, various electrodes such as grid terminals 23 are led out, which are, in turn, connected to a driver circuit (not shown) and the like. The anodes 14 are scanned with a time-division pulse signal and a positive display pulse in synchronism with the scanning is applied to the second control electrodes 15 desired so that the luminous dots 17 on the phosphor layers 13 may selectively emit light as desired.
In order to prevent leakage luminescence, negative voltage (cut-off-bias) is applied to the second control electrodes 15 corresponding to the phosphor layers 13 (or luminous dots 17) of which light emission is not desired. The cut-off bias produces negative potential which imparts repulsion to electrons to deflect them from their path, and the electrons fail to uniformly impinge on the luminous dots 17 which are designed to carry out luminous display. This results in a part of the luminous dots 17 failing in light emission. In order to eliminate such a phenomenon called eclipse or shading, the first control electrode 18 is applied thereto positive voltage sufficient to keep an electric field between the first control electrode 18 and the filamentary cathodes 20 substantially uniform.
When mounting the write head 10 on the above-described optical printer as a light source, the write head 10 is positioned adjacent to the photosensitive drum 1 as a record medium so that a direction of movement of the photosensitive drum 1 may be perpendicular to the longitudinal direction of the anode conductors 12. This causes the direction of arrangement of the luminous dots 17 defined by each of the slits 16 to be oblique with respect to an axial direction of the photosensitive drum 1. However, dot-like lights emitted from the luminous dots 17 can be continuously connected together so as to form a straight line parallel to an axis of the photosensitive drum 1 on a surface of the photosensitive drum by suitably selecting a width of each of the luminous dots 16 in the longitudinal direction of the anode conductors 12 (or the axial direction of the photosensitive drum 1) which is determined depending on a width of the slit 16 and suitably adjusting luminous timing of each of the luminous dots 17 depending on a rotational speed of the photosensitive drum 1 according to a suitable electrical signal treatment.
However, the conventional write head is disadvantageous in that the second control electrodes 15 are not only to determine a size of each luminous dot 17 and an interval between the luminous dots 17 but also to control actuation of the luminous dots 17. Accordingly, it is required to provide each of the second control electrodes 15 with the slit 16 and electrically insulate them from one another so that the second control electrodes 15 may attain both functions at the same time. Also, the second control electrodes 15 must be accurately positioned with predetermined intervals therebetween. More particularly, in order that lights emitted from the luminous dots 17 may reach to the photosensitive drum 1 to be aligned precisely so as to form a linear latent image like a straight line parallel to the axis of the photosensitive drum 1 on the surface of the photosensitive drum 1 for improving printing quality of the optical printer, a plurality of the second control electrodes 15 must be arranged in parallel to one another keeping the predetermined intervals on the substrate 11 with high precision and at precise inclination with respect to the anodes 14. However, it is highly difficult to precisely position the second control electrodes 15 electrically separate from one another at precise inclination. Also, even if such troublesome arrangement of a plurality of the second control electrodes 15 on the substrate 11 through the insulating layer is possible, it is indispensable to heat the substrate 11 and the whole envelope to a high temperature of 400.degree.-500.degree. C. when the second control electrodes are to be fixed on the insulating layer of the substrate 11 by means of glass adhesive or the envelope is to be sealedly mounted on the substrate 11. The heat treatment creates a difference in thermal expansion between the substrate 11 made of glass and the second control electrodes 15 made of metal, which causes deviation of the position and angle of the second control electrodes arranged separate from one another on the substrate 11 and results in a failure in precise positioning of the second control electrodes at predetermined intervals. Also, fixing of the second control electrodes 15 on the substrate 11 is carried out while the electrodes are expanded due to heating, and stress may occur in the electrodes 15 after the cooling. The stress acts on the second control electrodes 15 obliquely arranged with respect to the anode conductors 12, resulting in a failure in oblique arrangement of the electrodes 15 at a predetermined angle.
Such a failure in positioning of the second control electrodes 15 at precise intervals and angle causes lights emitted from the luminous dots 17 of each group defined by each of the slits 16 to fail to align in order so as to form a straight line on the surface of the photosensitive drum 1 or lights emitted from the luminous dots 17 of two groups defined by each adjacent two second control electrodes 15 to be discontinously projected on the photosensitive drum 1. This makes it impossible to form a continuous liner latent image like a straight line parallel to the axis of the photosensitive drum 1 on the drum, and the latent image will be dot-like lines. Thus, the printing quality cannot be improved to a desired level.
Also, the first control electrode is made of the single sheet material formed with a plurality of the slits 19. This permits the interval between the slits 19 to be precisely determined by photolithography, etching or the like. However, as explained hereinabove, it is highly difficult to precisely position the second control electrodes 15 so as to keep the intervals between the respective adjacent two second control electrodes 15 and those between the adjacent slits 16 constant. Accordingly, it is troublesome to adjust a position of the first control electrode 18 so as to vertically align the slits 16 and 19 with each other when the first control electrode 18 is arranged above the second control electrodes 15. In order to avoid such a problem in the conventional write head the first control electrode 18 is positioned and aligned with respect to the second control electrodes 15 so that the slits 16 of the second control electrodes 15 may be viewed through the corresponding slits 19 of the first control electrode 18.
The conventional write head has another disadvantage that the second control electrodes 15 are heated by a control current and thermally deformed when they are driven.
The write head for the optical printer is required to exhibit high luminance. In order that the write head exhibits high luminance, a high voltage must be applied to the second control electrodes 15. However, the second control electrodes 15 are to control actuation of the luminous dots 17 as well as define the luminous dots 17 by the slits 16. Accordingly, application of high voltage to the second control electrodes 15 requires a driver IC having high dielectric strength, which results in increasing manufacturing costs of the write head.